1. Field of the Invention
The present invention relates to a code transmitting method, and more particularly to a code transmitting method for transmitting a code having an error detection or correction code comprising a product code added thereto.
2. Related Background Art
In prior art digital code transmission, particularly in a field of magnetic recording, it was necessary to suppress a low frequency band component of a digital code to be recorded. For this purpose, for example, an 8-bit code is converted to a 9-bit code (8/9 conversion) in order to use only the code which does not include the low frequency component. In this method, however, the redundancy of the code is so high that the volume of data to be transmitted increases.
As a method which prevents the increase of the redundancy, a mapping coding method has been known. It is applicable only to a code sequence having a high correlation among codes, and the low frequency band component of the code sequence can be suppressed by making use of the correlation. For example, an input code is differentially encoded and a code in a vicinity of zero level is converted to a code having a small DSV (digital sum value) so that the low frequency band component of the code sequence is suppressed. For example, a 4/4 mapping coding method which converts a 4-bit differential code to a 4-bit code has been known.
However, since the mapping coding method makes use of the correlation of the code sequence, the suppression effect for the low frequency band component is not attained for the code having no correlation. For example, it is difficult for the error detection or correction code to suppress the low frequency band component by the mapping coding method. As a result, the suppression effect for the low frequency band component is low for the entire code sequence including the information code and the error detection or correction code. This causes the increase of a coding error is a decoding process.
FIG. 1 shows a format of a data frame of a code sequence of the type described above. In FIG. 1, information data is an information code sequence coded by the mapping coding method, and an error detection or correction code is a check point such as Hamming code or Reed-Solomon code.
FIG. 2 shows a data matrix of a product code comprising inner codes provided in the data frame and outer codes provided by arranging the data frame in lengthwise. In this arrangement, since check codes are arranged two-dimensionally, it is suitable to handle image data.
However, assuming that the respective rows (respective data frames) are sequentially transmitted in the data matrix of FIG. 2, the suppression effect for the low frequency band component by the mapping coding method is not expected because there is no correlation of data in a portion at which the inner code and the outer code are continuously transmitted. Particularly in a data frame which comprises only the check points of the outer code and the inner code, the check points continuously appear for a long time. As a result, the suppression effect for the low frequency band component of the code sequence in the vicinity of the check points is materially lowered.
In the Japanese Patent Application No. 60-169420 filed by the assignee of the present invention, a method for suppressing the low frequency band component by distributedly arranging the check points of the data frame in the information codes of the respective data frames as shown in FIG. 3, is disclosed.
Further, in the Japanese Patent Application No. 60-291172 filed by the assignee of the present invention, a method for switching rows in the data matrix to which the outer codes of the product code are to be added, is disclosed.
However, in the method of distributedly arranging the check points in the information codes of the data frames, no solution is attained for the outer codes of the product code as shown in FIG. 2. Thus, in a data frame which comprises only the check points of outer codes and the inner codes, the suppression effect for the low frequency band component of the code sequence is materially lowered.
In the method of switching the rows of the data matrix to which the outer codes of the product code are to be added, the above problem may be solved but a method for generating the outer code must be changed from column to column, and it is also necessary to change a start point of coding of the outer code in addition to generate the outer code. Particularly when there are many check points of the generated outer codes, it is necessary to select a pattern which avoids the continuation of the check points of the outer codes along the row direction. As a result, a complex process is required.